The present invention relates generally to the transmission and distribution of electrical power, and particularly to power substations of the type having substation components within a metal enclosure pipe containing an insulating gas and in which one of the components in the enclosure pipe is a voltage surge arrester.
Voltage surge arresters are sometimes incorporated inside a metal enclosure pipe of a gas insulated power substation installation. The use of surge arresters in such apparatus is described for instance in the following U.S. Pat. Nos: 3,624,450 issued to H. W. Graybill 30 Nov. 1971 and 3,753,045 issued to Osmundsen et al. 14 Aug. 1973. Further details of such substations are found for example in the technical article entitled Where Land is Scarce and $$ High Consider the SF.sub.6 Mini Sub, Transmission and Distribution, December 1973, pages 34-38 and 109.
One problem with arresters incorporated in such a gas insulated installation is presented by the possibility of a violent failure of the arrester. Because the insulating gas in the system, generally sulphur hexaflouride (SF.sub.6), is not suitable for the arcing that occurs in an arrester during its operation, arresters, even when enclosed in gas insulated systems, are nevertheless usually provided with a porcelain housing to isolate the internal components of the arrester from the insulating gas in the system. Should such an arrester for some reason fail, so that internal flashover permits a very substantial current flow through it to ground, there would be generated inside the porcelain housing of the arrester a considerable amount of arc-generated hot gas. If this gas is not vented from the inside of the arrester, it can result in a violent fracturing of the porcelain housing, pieces of which may rupture the metal enclosure pipe of the system and possibly pose a hazard to personnel in the vicinity.
Heretofore, there have been provided means for venting such gases generated inside the arrester to the outside of the gas insulated system by, for example, providing a diaphragm at the bottom mounting of the arrester which will rupture under gas pressure to allow the gas to escape to the outside air. Such arrangements are described for example in the above-cited patents. Venting of the gases to the outside air has been thought desirable in order to prevent contamination of the insulating gas inside the system by the arc-generated gases. One difficulty with this approach, however, is that although the gases generated inside the arrester are indeed released through the bottom of the arrester to the outside air, current will continue to be passed through the failed arrester to ground. Under certain internal flashover conditions, the energy absorbed in the arrester in the failure mode is so great that a violent fracture of the arrester porcelain can occur in spite of such venting of gases from the bottom to the outside. Moreover, thermal shock alone can cause fracture of the porcelain and venting of the SF.sub.6 to the air. This venting in itself poses a hazard, in that some toxic products are formed by electric arcing in SF.sub.6.